Sea Ice Trends and Climatologies from SMMR and SSM/I-SSMIS, Version 2

NSIDC provides this data set to aid in the investigations of the variability and trends of sea ice cover. Ice cover in these data are indicated by sea ice concentration: the percentage of the ocean surface covered by ice. The ice-covered area indicates how much ice is present; it is the total area of a pixel multiplied by the ice concentration in that pixel. Ice persistence is the percentage of months over the data set time period that ice existed at a location. The ice-extent indicates whether ice is present; here, ice is considered to exist in a pixel if the sea ice concentration exceeds 15 percent. This data set provides users with data about total ice-covered areas, sea ice extent, ice persistence, and monthly climatologies of sea ice concentrations.

Detailed Data Description

NSIDC provides this data set to aid in the investigations of the variability and trends of sea ice cover. Ice cover in these data are indicated by sea ice concentration: the percentage of the ocean surface covered by ice. The ice-covered area indicates how much ice is present; it is the total area of a pixel multiplied by the ice concentration in that pixel. Ice persistence is the percentage of months over the data set time period that ice existed at a location. The ice-extent indicates whether ice is present; here, ice is considered to exist in a pixel if the sea ice concentration exceeds 15 percent. This data set provides users with data about total ice-covered areas, sea ice extent, ice persistence, and monthly climatologies of sea ice concentrations.

Total Ice-Covered Areas and Sea Ice Extent Data

Along with one historical data file from the Electrically Scanning Microwave Radiometer (ESMR), these data are derived from the Scanning Multichannel Microwave Radiometer (SMMR), the Special Sensor Microwave/Imager (SSM/I), and the Special Sensor Microwave Imager/Sounder (SSMIS).

Daily and Monthly Total Ice-Covered Area and Total Sea Ice Extent Data

Monthly mean sea ice concentrations were used to derive monthly extents for both the NASA Team and Bootstrap algorithms. Monthly ice concentration extents are provided because they will tend to average out the effects of storms and other short-term events on the location of the ice edge. However, this has not been verified by NSIDC.

Regional Graphs: Time-Series Plots

The total ice-covered area and sea ice extent data are used to create .png images of regional graphs showing monthly time-series plots of ice-covered areas, area anomalies, ice extent, and extent anomalies for both the NASA Team and Bootstrap algorithms. The methods and sources for how these regions were geographically defined are discussed in Parkinson and Cavalieri (2012), Cavalieri and Parkinson (2008), and Parkinson et al. (1999).

Ice Persistence

Monthly climatologies of ice persistence binary data files with corresponding .png images are intended for users interested in persistence of ice over a particular month during the time series, which includes January 1979 through current processing. Ice persistence fields provide information on how frequently ice occurs in a region during a given month over the time period of the data. The ice extent climatologies are derived each month from monthly-averaged sea ice concentrations using the NASA Team data.

Monthly Climatology of Sea Ice Concentration

Monthly sea ice concentration climatology binary data files with corresponding .png images represent mean ice concentration percentages for each month over the entire time period of January 1979 through current processing. The data are derived from the NASA Team data.

esmr-smmr-ssmi-merged/ - this folder contains ASCII text data files of daily and monthly sea ice extent summaries from 01 January 1972 to 31 December 2002 for the Northern Hemisphere and 01 January 1973 to 31 December 2002 for the Southern Hemisphere

Total Ice Area Extent Data Files

Bootstrap and Nasateam Files

This section explains the file naming convention used for these files with an example. The following file naming conventions pertain to both the NASA Team and Bootstrap algorithm data files. Data are provided in two different temporal resolutions: daily and monthly.

There are two different types of daily files: ice covered area files and sea ice extent files.

Monthly anomalies of ice-covered area and ice extent are calculated from monthly mean ice concentrations throughout the period of study. These anomalies show the deviations from the mean monthly values averaged over the entire time series.

The remaining columns of the Arctic and Antarctic daily area and daily extent files list the total ice-covered area (km2) and total sea ice extent (km2), respectively. The columns are labeled by region. Refer to Table 6 and Table 7 for a list of abbreviations for the column names for the Arctic and Antarctic regions.

Table 6 lists the Arctic regions covered by this data set along with their abbreviations found in the data files. Table 7 lists the Antarctic regions covered by this data set along with their abbreviations found in the data files.

Parameter Description

Ice Persistence

The ice persistence data indicate the historical frequency of the presence of ice with at least 15 percent concentration in each pixel of the 25 km polar stereo grid in a binary file.

The Northern Hemisphere binary files are 304 x 448 byte data.

Southern Hemisphere binary files are 316 x 332 bytes.

Table 10 lists the Data Values for the ice persistence files:

Value

Description

Table 10. Data Values for the Ice Persistence Files

0

Water, no ice

1 - 100

Historical frequency of the presence of ice, as a percentage

253

Coastlines (land adjacent to water)

254

Land

Monthly Climatology of Sea Ice Concentration

The monthly climatology data give the average sea ice concentration for each polar stereo pixel. A minimum concentration value of 15 percent is imposed. When concentrations are lower than this, the grid cell is considered to be free of ice.

Total Ice-Covered Areas and Sea Ice Extent Data

The daily and monthly total sea-ice extent and sea-ice area data for specified Arctic and Antarctic regions are displayed in ASCII text tables. Anomalies are also provieded, which indicate departures from the long-term averages.

Sample Data Images

Total Ice-Covered Areas and Sea Ice Extent

Refer to Figures 1 and 2 for Bootstrap sample data images for the Total Ice-Covered Areas and Sea Ice Extent data files.

Total Ice-Covered Areas and Sea Ice Extent Data

In computing the total ice-covered area and ice extent data with both the NASA Team and Bootstrap Algorithms, pixels must have an ice concentration of 15 percent or greater to be included. Total ice-covered area is defined as the area of each pixel with at least 15 percent ice concentration multiplied by the ice fraction in the pixel (0.15 to 1.00). Total ice extent is computed by summing the number of pixels with at least 15 percent ice concentration multiplied by the area per pixel; thus, the entire area of any pixel with at least 15 percent ice concentration is considered to contribute to the total ice extent. Anomalies are the difference between the current time period's average value and the long-term average. A positive anomaly indicates that the current period has greater extent or area than the average

Ice Persistence Data

Individual ocean pixels containing a minimum of 15 percent ice cover are summed throughout the time series. The maximum ice persistence value is 100 percent. Each pixel represents the percentage of years for which ice was present during that month. Only the NASA Team Algorithm is used to compute these data.

Monthly Climatology of Sea Ice Concentration

A threshold of 15 percent concentration is applied to the monthly climatology fields. Only the NASA Team Algorithm is used to compute these data.

In the northern hemisphere, very small differences in sea ice extent were found: less than 0.2 percent for the NASA Team algorithm and less than 0.05 percent for the bootstrap algorithm.

Differences in northern hemisphere sea ice area were also small. Most years had less than 0.05 percent. Differences in the Bootstrap sea ice area were generally less than 0.01 percent.

For both the Bootstrap and NASA Team algorithms, the differences in the southern hemisphere were negligible.

These small differences are found in the persistence and monthly climatology data also. The northern hemisphere monthly fields showed no difference in 95-98 percent of the sea ice values, and fewer than 0.02 percent of pixels had differences in ice concentration of greater than 2 percent.

Reprocessing is complete and updated data are now available through 31 December 2013 for all Sea Ice Trends and Sea Ice Trends and Climatologies from SMMR and SSM/I-SSMIS data sets.

1

04 September 2013

Reprocessing is complete and updated data are now available through 31 December 2012 for all Sea Ice Trends and Sea Ice Trends and Climatologies from SMMR and SSM/I-SSMIS data sets. With this update, the following changes have been implemented:

Total Ice-Covered Area and Extent

The cutoff value which defines a region as ice-covered or not ice-covered has been adjusted from 14.8 percent to 15 percent to more closely match the methodology used to calculate other sea ice extent and area products at NSIDC.

To prevent inconsistencies between different algorithms, these parameters are now calculated exclusively using the NASA team algorithm.

Monthly Ocean Masks and Maximum Extent Masks

The ocean mask files and maximum extent mask files were removed as they are not the masks used in the production of these data sets.

This most recent reprocessing also rectifies two previous releases of these data sets that contained erroneous data. In January/February of 2012, and May of 2013, the products were to have contained data from 2007 to 2010 and from 2010 to 2011, respectively. However, the climatology products contained data only through 2007. We recommend replacing previously downloaded 2010 and 2011 climatology data files with the climatology files that now extend through 2012. The Total Ice-Covered Area and Extent data set did contain post-2007 data, but reprocessed data utilizing the new methodology are available in the 2012 release.

Missing Data and Monthly Anomalies

Periods for which there are no available data are described in Table 13.

Monthly anomalies of ice-covered area and ice extent are calculated from monthly mean ice concentrations throughout the period of study. These anomalies show the deviations from the mean monthly values averaged over the entire time series.

Table 13. Missing Data

Time Periods

Data Type

Description

October 1978, December 1987

Sea ice

Sea ice data are missing for most days, so monthly values are not calculated for these months.

January 1988

Sea ice

January 1988 is missing data for the first half of the month. A value is given for this month, but the value may not represent true monthly mean since the sea ice data are incomplete.

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Theory of Measurements

There is a circular section over the Northern Hemisphere pole known as the pole hole, which is never measured due to orbit inclination. For the purposes of ice extent, pixels under the pole hole are always considered to be at least 15 percent. For total ice-covered area, the pixels under the pole hole are not used. The Southern Hemisphere also has a pole hole, but it does not affect this sea ice data set because there is only land under this hole. For SMMR, the hole is 611 km in radius and is located poleward of 84.5 degrees North. For SSM/I and SSMIS, the hole is 311 km in radius and is located poleward of 87.2 degrees North.

The difference in pole hole areas between SMMR and SSM/I-SSMIS results in a discontinuity in the Northern Hemisphere ice-covered area time series across the instrument transitions.

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Sensor or Instrument Description

For sensor or instrument description information, see the following Web pages:

Contacts and Acknowledgments

Document Information

DOCUMENT CREATION DATE

January 2017

DOCUMENT REVISION DATE

No technical references available for this data set.

FAQ

What is the difference between the NASA Team algorithm and the Bootstrap algorithm?

For general analyses or creation of simple images, either algorithm will suffice. The Bootstrap sea ice concentration data set is believed to be more useful for modeling and process studies in the polar regions because it is generally free of residual errors that could not be removed by... read more

How To

How to import the sea ice trends data into ArcGIS

The following are instructions describing how to import "Sea Ice Trends and Climatologies from SMMR and SSM/I-SSMIS" binary files into ArcGIS. These instructions were tested with ArcInfo 10.0.
1. Rename the Data File.
In order to be able to import a Polar Stereographic monthly climatology sea... read more